Bitumen or asphalt is the heaviest portion from the oil distillation process. Due to the different origins and distillations processes of such oils, the resulting bitumen may have a wide range of properties and characteristics. In the present invention, bitumen refers not only to the product from oil by direct distillation or from distillation of oil at reduced pressures, but as well the product coming from the extraction of tar and bituminous sands, the product of oxidation and/or fluxation of such bituminous materials, as well as blown or semi-blown bitumens, synthetic bitumens (such as described in FR 2 853 647 A1), tars, oil resins or indene-coumarone resins mixed with aromatic and/or paraffinic hydrocarbons and the mixtures of such.
The main application for bitumen is in asphalt mixtures where the bitumen is mixed with mineral aggregates that can be of different size, shape and chemical nature. These asphalt mixtures are particularly used for construction or maintenance of sidewalks, roads, highways, parking lots or airport runaways and service roads and any other rolling surfaces. In the present invention mineral aggregates are the product from quarries as well as aggregates recuperated from previous asphalt mixtures (as described in the AFNOR XP P98-135, December 2001), products from building demolition and their mixtures. Other common components in asphalt mixtures are organic and inorganic fibers, such as glass, metal or carbon fibers, as well as, cellulose, cotton, polypropylene, polyester, polyvinyl alcohol and polyamide fibers.
The mixing process for the fabrication of asphalt mixtures using the composition described in this invention, can take place by different methods that may be grouped into three main categories based on their fabrication temperatures: processes at room temperature, processes with a fabrication temperature above 100° C. and processes at temperatures between room and 100° C.
The asphalt mixture fabrication processes taking place at room temperature, depend on methods that provide enough fluidity to the bitumen at such temperatures. One of such methods can be, for example, the one based on the addition of volatile solvents to the bitumen. This would allow the proper coverage of the aggregates by the solubilized bitumen at room temperature, and its proper laying and compaction. However, large quantities of volatile solvents are required which pollutes the atmosphere when evaporated. This technique is pretty much vanished since the use of volatile solvents can be avoided by the use of other techniques.
Another process that allows the production of an asphalt mixture at room temperatures conditions is the one that uses an emulsion or dispersion of bitumen in water as a mean to make it fluid. The asphalt mixtures fabricated by this process have the advantage that no thermal treatment is required for the aggregates and practically no polluting emissions are generated. This process can be combined with the technique mentioned above which uses volatile solvents added into the bitumen prior to the emulsification or dispersion. Nevertheless, the mechanical properties obtained by this method are, in general, lower that the ones obtained by hot mixing methods, where the aggregates are used dried after a thermal treatment above 100° C. Bitumen emulsions and dispersions are commonly used for example to produce grave emulsion, storable cold mixes, cold wearing courses like microsurfacing or like the ones produced with pugmills, transported with haul trucks and compacted with rollers and to waterproof surfaces.
The most commonly used method for the fabrication of asphalt mixtures is the one that takes place at temperatures above 100° C. At these temperatures, the bitumen can be fluid enough to properly cover the aggregate that is, in contrast to the previous methods at room temperature, dried during the heating process. The obtained hot asphalt mixtures have also to be laid and compacted at elevated temperature to guarantee their fluidity. The temperature at which the aggregates are heated is chosen to obtain proper evaporation of their moisture and to ensure an adequate fabrication temperature for the used bitumen. The fabrication temperature is set mainly by the viscosity of the utilized bitumen, the more viscous the bitumen the higher the fabrication temperature.
For example, in France, an asphalt mixture using bitumen with a penetration of 35/50 is generally fabricated at a temperature of 150° C. to 170° C. and laid at 140° C. (as recommended by Eurobitume). This hot asphalt mix process is widely used due to its simplicity and robustness, since the main parameter to control is the fabrication temperature. However, a substantial amount of heat goes to the heating and drying of the mineral aggregates (˜90-96 wt % of asphalt mixture), which makes this an energetically expensive process that also releases a significant amount of undesired emissions.
Recently, there have been several developments for asphalt mixture fabrication at temperatures above room temperature but below 100° C. Examples of such processes are: the use of two different types of bitumen during fabrication (as in WO 97/20890), the introduction of a fraction of cold and wet aggregates during the mixing stage to create a fluid bitumen foam (as in EP 1 469 038 and EP 1 712 680), or the use of a bitumen/water emulsion to also produce a foam during mixing to achieve the aggregate covering (as in WO 2007/112335). These processes have several advantages, in particular on the reduction of energy consumed and polluting emissions generated, but they require substantial modification to the standard asphalt hot mixing equipment.
Other techniques exist and are known to those skilled in the art, like tack coat, cheap seal, surface dressing employing anhydrous bituminous binder or bitumen emulsions. The anhydrous bituminous binder or bitumen emulsion has to achieve cohesion very fast once sprayed over the surface to be treated so that the aggregates are not expelled upon mechanical action. One conventional solution is to use a volatile solvent mixed with the bitumen so that the solvent evaporation enables a cohesion increase. This is no longer seen as a long-term solution because of the related organic emissions. FR 2 768 150 proposes to use, admixed with bitumen, a non-volatile solvent that chemically reacts in contact with the oxygen of the air to get the right bitumen cohesion.
However, the cohesion-augmentation kinetics remains difficult to control and use is made of a metallic catalyst.
It is known by any person skilled in the art that polymers can be added to the bitumen in order to fabricate asphalt mixtures with enhanced mechanical properties. Polymers are large molecules formed by the chemical linkage of several repeating units or monomers. Modification of bitumen with polymers of high molecular weights (above ˜10,000 g/mol) is generally required to improve the mechanical behavior of an asphalt mixture because the mechanical properties of bitumen are susceptible to temperature changes.
Although, there exist solutions to harden bitumen, that is, to increase the stiffness of the bitumen at high service temperature to avoid rutting, like for example by incorporating paraffins or polyphosphoric acid, these solutions are only partial because the high susceptibility of the modified bitumen remains, creating or even enhancing problems at low temperature like for example a lack of flexibility.
Hence polymer modification of bitumen is very often used to increase its low temperature flexibility, that is below the ambient temperature and down to about −40° C., and the same polymer modification increases the softening point of the bitumen. It can also increase the cohesion and stiffness of the bitumen at high service temperature and consequently that of the asphalt mixture made with it, improving its resistance to rutting. Examples of polymers commonly used in the modification of bitumen are: styrene butadiene rubbers, styrene/butadiene block copolymers, ethylene vinyl acetate copolymers, polyethylene and other alpha-polyolefins (see “Asphalt binder testing manual”, Asphalt Institute 2007). The use of non-crosslinked low molecular weight polymers, also known as oligomers, or other small molecules cannot modify the bitumen in the manner as large polymers do.
Incorporating polymers into the bitumen, even at the low contents commonly used (from 2% to 6% of polymer) is not an easy task. Polymers and bitumen have at most only a partial compatibility that usually makes the polymer to phase separate from the bitumen overtime. In addition, due to the high viscosities of molten polymers, the mixing process requires high temperatures and long mixing times under vigorous agitation to achieve a good dispersion of the polymer in the bitumen. The required temperatures to achieve the dispersion of polymer into bitumen are generally higher than the usual storage and fabrication temperatures according to the nature of bitumen.
For example, U.S. Pat. No. 5,618,862 shows as an example that the dispersion of a styrene butadiene copolymer with a molecular weight of 100,000 daltons at 3.5% in bitumen with a penetration of 80/100 takes 2.5 hours at 175° C. to be homogeneous. For this kind of bitumen, a typical storage temperature is between 140 and 160° C. The higher temperatures required for dispersing the polymer in the bitumen may also prove detrimental to the bitumen properties since it would accelerate its oxidation. The addition of a crosslinking agent, such as a sulfur-containing compound, is also commonly used in polymer-modified bitumens to further increase the molecular mass of the dispersed polymer by forming a chemical network between the preexisting polymer molecules. Such network increases the viscosity of the bitumen even further but avoids phase separation. In addition, such sulfurous compounds combined with the high temperatures required to achieve the mixing present important safety issues during the modified bitumen fabrication process. The difficulty to prepare modified bitumen with polymers, make the process accessible only to bitumen refiners or large construction companies which are the only ones capable of making the considerable investment in the adequate mixing equipment.
The use of a modified bitumen in order to fabricate an asphalt mixture generally results in a change of the fabrication process relatively to the unmodified bitumen. In the case of emulsions, for example, the addition of polymer may limit the grade of bitumen used since the emulsification process with water limits the temperature at which the bitumen can be added. More viscous bitumen grades may be used to fabricate emulsions in water at higher than atmospheric pressures. However, this adds a certain complexity to the emulsion fabrication process. In the case of hot mix asphalt fabrication, higher temperatures for fabrication, laying and compaction are required when modified bitumen is used. The higher viscosity of polymer-modified bitumen, compared to the one of the pure bitumen, can also bring problems to processing methods at temperatures below 100° C. and higher than room temperature, reducing the overall asphalt mixture fluidity.
It is of practical interest if the modification of bitumen by a polymeric material can be done without a substantial increase in its fabrication temperature, compared to the pure bitumen, while still obtaining an enhancement in mechanical properties on the resulting asphalt mix.
Also, a temperature reduction during the polymer dispersion and asphalt mixture fabrication process is of practical interest because it would lead to several advantages. A reduction in the dispersion temperature, and/or in time, reduces the amount of bitumen oxidation and aging, extending the life of the final application, such as in an asphalt mix for a road. If such reduction in temperature is translated to the asphalt mixture fabrication process, it would reduce the amount of energy consumed during dispersion and, most importantly, during an asphalt mixture fabrication process. Decreasing the aggregates and bitumen temperatures during the asphalt mixture fabrication process will also significantly reduce the amount of polluting emissions, including CO2 and other greenhouse effect gases.
There are several approaches to reduce the time and temperature necessary for the dispersion of polymers in bitumen. One of such approach is the addition of a solvent during the mixture, which can also be used to disperse the polymer before adding to the bitumen. The use of volatile solvents, as discussed above, is not a practical option due to the polluting effect and difficulty of using them at elevated temperatures. Other solvents could be used such as vegetable oils or their derivates. The use of such solvents in the production of modified bitumens for asphalt mixtures often leads to a softening of the material and an increase in rutting.
WO 2005/087869 describes a composition of polymer-modified bitumen using a mono-alkyl ester of a vegetable oil as solvent to facilitate mixing. In addition, an amide additive is also added to overcome the decrement in mechanical properties due to the addition of the alkyl ester. However, the addition of about 6% of rapeseed oil methyl ester, to previously dissolve the polymer as stated in one of the examples, would still make the mixture softer, resulting in a softer asphalt mixture. In addition, a temperature higher than 160° C. and 30 minutes of stirring is still required to incorporate the polymer solution to the bitumen with a penetration of 160/220. A typical storage temperature for this kind of bitumen when pure is between 130° C. and 150° C.
U.S. Pat. No. 6,156,113 describes another approach to enhance the mechanical properties of the final bitumen composition while maintaining low viscosities at fabrication temperatures. In this patent, fatty acid monoesters are added to the bitumen to reduce viscosity by a solvation effect at fabrication conditions while, by the addition of a metal catalyst, crosslinking of such esters takes place under application conditions. This process may take several days. Although this method allows for low viscosities at fabrication conditions and enhanced mechanical properties of the final asphalt mixture, the use of certain metal catalysts may be restrained due to their negative impact to environment and men.
FR 2 871 804 proposes the use of a polymer-bitumen mixture containing a high level of polymer, called a master batch. This mixture is prepared with an extrusion device and then diluted with bitumen to get the right polymer dosage in a short time. The drawback of the solution is that a special device is still necessary to mix the polymer and the bitumen to produce the master batch, resulting in a considerable economic investment.
Since the main problem with the addition of regular or conventional polymers into bitumen is their high viscosity, one solution would be to have a polymer with good mechanical properties at asphalt mixture application temperatures (about −20° C. to 70° C.) while having very low viscosity at elevated temperatures (above 100° C.). Such low viscosity at higher temperatures would make the dispersion of such polymer into the bitumen considerably easier under lower temperatures, milder mixing conditions and shorter mixing times. This would also result in an easier use of the modified bitumen thanks to the lower temperatures and or lower process duration.
Polymeric materials with such properties can be achieved by the use of oligomers or monomers than assembly into a supramolecular polymer-like structure with non covalent bonds at low temperatures but dissociate at high temperatures.
WO 01/07396 describes a polymer like material comprised of oligomers that can associate into large structures by means of hydrogen bonding between specific carboxylic acid and alcohol functional groups. The resulting material shows mechanical properties far superior to those of the original monomer, which increase with the number of associated functional oligomers. No application with bitumen is discussed in this text.
WO 03/059964 describes another supramolecular polymer based on a different chemistry. In this case, polymer-like properties are also achieved by the interconnection of the smaller molecules by hydrogen bonding. No application with bitumen is discussed in this text.
WO 2006/087475 describes an elastomeric material formed by the supramolecular assembly of smaller molecules. The rubber-like material of this invention becomes a liquid above a certain temperature due to the dissociation of the hydrogen bonds. The transition from elastic polymer to liquid is reversible in temperature. No application with bitumen is discussed in this text.